Conventionally, there is known a parallel-type hybrid vehicle provided with an engine and a regenerative motor, wherein the motor is driven in accordance with a requested output, with the wheels being driven mainly with use of the engine (see e.g. Patent Literature 1). The hybrid vehicle is configured such that in the operating range where the required output of the vehicle is low, and where the specific fuel consumption of the engine is very high, the engine is stopped and the wheels are driven solely by the motor; and in the other operating ranges, for instance, in the operating range where the specific fuel consumption is low, the engine is driven so that the engine and the motor are used in combination for improving the specific fuel consumption. Meanwhile, the hybrid vehicle is configured such that when the vehicle is decelerated, kinetic energy is converted into electric power (regenerative electric power) by a regenerative braking operation of the motor to accumulate (collect) the electricity. Thus, electric power for driving the motor is secured.
In the hybrid vehicle having the above configuration, the motor is coupled to a driving shaft such as a propeller shaft via a transmission. Thus, the hybrid vehicle incorporated with an IPM motor (Interior Permanent Magnet Synchronous) motor such as a three-phase induction motor has an advantageous configuration. Specifically, it is difficult for the IPM motor to generate a torque in a high rotational speed range because a counter electromotive force is generated due to the structure of the IPM motor. However, coupling the IPM motor to a transmission as described above for restricting the rotational speed range of the motor makes it possible to generate a torque in a wide speed range, in other words, makes it possible to output and regenerate power.
In the conventional hybrid vehicle in which a motor is coupled to a driving shaft via a transmission, however, during a braking operation when the vehicle runs at a high speed, regenerative electric power to be collected by the motor is restricted in a certain rotational speed range due to the existence of the transmission, regardless that the wheels are rotated at a high speed. In other words, the conventional art may fail to efficiently collect kinetic energy of the high-speed rotating wheels as regenerative electric power. Further, in the conventional hybrid vehicle, kinetic energy is input to the motor via the transmission. This may cause energy loss. Thus, it is difficult to efficiently collect kinetic energy as regenerative electric power.